In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of optical fiber comprising a transmission link of a WDM fiber optic network. At a receiver end of the transmission link, the optical channels are separated, whereby an optical receiver can detect each optical channel.
Light tends to lose power when propagating through an optical fiber. Yet, some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
A rare earth doped optical fiber such as an erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical amplifiers employed in many modern fiber-optic networks. A single EDFA module can amplify up to about a hundred optical channels at a time, thus providing significant cost savings. When continuous wave signals are transmitted, for example in PON systems conventional optical amplifiers are typically used. However in systems where the distances between and optical line terminal (OLT) and each optical network unit (ONU) differ, the OLT must be able to receive optical burst signals with different intensities from the ONUs. Furthermore the PON repeater based on optical amplifiers must be able to amplify these signals without any distortion. Unfortunately, optical burst signal amplification leads to optical surges, which may well cause failure of the optical receiver as well as interfering with the reception of normal signals at the OLT due to gain dynamics.
As a result burst-mode optical amplifiers are required which suppress these optical surges and provide gain stabilization.
Much work has been done to lessen the effects or reduce an EDFAs transient response and one method of lessening the unwanted effects is by gain clamping. Using EDFAs in burst mode is highly desirable, for example in communication systems such as NG-PON2.
There are numerous patents and publications related to gain clamped optical fiber amplifiers but they tend to be costly and none are perfect solutions to transient effects, especially those caused by burst-mode transmission.
It is an object of this invention to provide a cost effective optical amplifier, which does not require costly complex pump control or feedback circuitry to control output pump power.